Boron is important in ensuring normal tissue development in plants. It is important in the development of the fine structure of cell walls, in facilitating sugar translocation and in the synthesis of nucleic acids.
In apple, boron deficiency causes internal and external cork development and is of frequent occurrence in orchards in North America. In the UK, boron deficiency in apples is quite rare and is associated with boron concentrations in the fruit of less than 0.1 mg 100g-1 fresh weight giving rise to internal corking symptoms.
Whilst it is important to ensure that apple trees receive sufficient boron, excessive levels can cause earlier maturation of the fruit and increased incidence of water core at harvest and may promote the development of internal breakdown and decay in stored fruit.
There is evidence that a single ‘Solubor’ (Borax Consolidated Ltd) spray at petal fall or 2-4 weeks later reduces skin cracking in Egremont Russet and in some orchards of Cox. There was also some indication that boron sprays could provide some measure of control of bitter pit in Egremont Russet. In other trials, ‘Solubor’ sprays applied to Discovery and Cox trees enhanced red colour development on the fruit but increased the incidence of skin cracking and flesh breakdown during storage.
Surveys of Cox and Bramley orchards have indicated that very few are marginal or deficient in boron. In view of the potential adverse effects of excess boron in the fruit, it is advised that growers arrange for an analysis of leaves and fruits in orchards where boron deficiency is suspected.
Boron deficiency can be corrected by soil application of borax (20 kg ha-1) or ‘Solubor’ (10 kg ha-1) in the spring. However, the relative immobility of boron in plants favours the use of foliar sprays as a means of correcting deficiency.
Recent work on Bramley showed that autumn sprays of boron were most effective in increasing boron in the flowers that developed in the following spring. Applications made at full bloom had no effect on flower boron concentrations but sprays applied during cell division increased the boron status of spur leaves (June) and of the fruit (July and harvest).
This work tends to confirm advice to apply 3 sprays of ‘Solubor’ (2 kg 1000 l-1 ha-1), with an appropriate wetter, starting at petal fall and repeated at 2-3 week intervals (MAFF, 2000). Other proprietary boron formulations can be substituted for ‘Solubor’ as appropriate.